is this TN-S or PME?

[THRIPSTER]

Thanks Westie - that's interesting. How do we reconcile the 1 sq inch in the street and size of the service into the house with the requirement to fit 25mm tails between meter and CU?

Regards

 

[EFLImpudence]

How do we reconcile the 1 sq inch in the street and size of the service into the house

Westie wrote 0.1sq.in. (64sq.mm.)

with the requirement to fit 25mm tails between meter and CU

It's NOT a requirement.

Table 4D1A.

 

[westie101]

It's NOT a requirement.

Wot EFLI said.

 

[THRIPSTER]

Didn't phrase that very well. What size tails are required for a 100A head? If the answer to that is 25mm then how come the service cable into the house can be so much smaller, survive and be perfectly adequate?

Regards

 

[EFLImpudence]

Because the BS7671 ratings are very conservative.

I believe the DNOs use 16mm² for 100A supplies.

 

[JohnW2]

Didn't phrase that very well. What size tails are required for a 100A head? If the answer to that is 25mm then how come the service cable into the house can be so much smaller, survive and be perfectly adequate?

It's not a satisfactory answer in terms of science/engineering, but the practical answer is that the regulations which apply on the two sides of the cutout/meter differ in their views of what size cable 'can survive and be perfectly adequate' when carrying a certain current. We don't often hear of the DNO's cables coming to thermal grief, so I guess BS7671 is just more (some might say 'over') cautious/conservative.

Kind Regards, John

 

[THRIPSTER]

Thanks EFLI and John. Doesn't the over sizing on the consumer side represent an outrageous waste of resources? Shouldn't the regulations which cover one side of the head also cover the other? And doesn't it make a mockery of the maximum demand calculations on the consumer side if the DNO simply assumes (in this case) 1.5kw per household to calculate their demand/cable sizes? Incidentally, I'd like to know over what period the 1.5kw per household figure is calculated. Is the answer because cables in the ground can dissipate the heat more easily?

Regards

 

[JohnW2]

Thanks EFLI and John. Doesn't the over sizing on the consumer side represent an outrageous waste of resources?

I suppose that depends upon whether one regards it as 'over sizing' or 'sensible safety margin'.

Shouldn't the regulations which cover one side of the head also cover the other?

That's obvioulsy a matter of opinion. The fact is that they don't!

And doesn't it make a mockery of the maximum demand calculations on the consumer side if the DNO simply assumes (in this case) 1.5kw per household to calculate their demand/cable sizes?

They don't use that figure for calculation the size of their supply cable - if they did, they'd probably be using 1mm² (or smaller, if they could find it ).

Incidentally, I'd like to know over what period the 1.5kw per household figure is calculated.

Others will probably have a definitive answer, but I suspect that it's over a full 24 hours, and probably averaged over a whole year (or, if they are a bit more 'conservative', over 24h in mid-winter!).

Is the answer because cables in the ground can dissipate the heat more easily?

A lot of supplies are overhead, hence not really that different from much wiring within installations.

Kind Regards, John

 

[EFLImpudence]

Thanks EFLI and John. Doesn't the over sizing on the consumer side represent an outrageous waste of resources?

Oh, don't go there.
A lot of electricians are routinely replacing adequate 16mm² tails with 25 'because they do', 'for future proofing', 'because that's what they carry' or similar 'reasons' and get quite put out when told it is a waste.

Shouldn't the regulations which cover one side of the head also cover the other?

They don't.

And doesn't it make a mockery of the maximum demand calculations on the consumer side if the DNO simply assumes (in this case) 1.5kw per household to calculate their demand/cable sizes?

Diversity, I suppose.

Incidentally, I'd like to know over what period the 1.5kw per household figure is calculated.

?

Is the answer because cables in the ground can dissipate the heat more easily?

I wouldn't think so as 'buried in masonry' has a slightly lower rating than 'in free air'.

 

[westie101]

And doesn't it make a mockery of the maximum demand calculations on the consumer side

As stated it is the ADMD (After Diversity Maximum Demand) figure. To get to this figure read the maximum demand on one of our tranformers on a long term basis, divide by the number of properties fed from that transformer.
That is the sort of figure you will get!

yes ratings are hugely different
e.g. a 16mm2 copper core split concentric is rated at:
in the ground 128A
in a duct 94A
in air 101A.

 

[JohnW2]

e.g. a 16mm2 copper core split concentric is rated at:
in the ground 128A
in a duct 94A
in air 101A.

That's interesting. The free air figure is not all that different from the BS7671 figure (94A), but the other two are appreciably higher than per BS7671. It's also interesting that, in contrast to the above, BS7671 gives the highest CCC for 'free air'.

Kind Regards, John

 

[RF Lighting]

Don't forget the DNO are working to 90° figures, and generally we work with 70° figures.

 

[JohnW2]

Don't forget the DNO are working to 90° figures, and generally we work with 70° figures.

That's true, and I guess that may make the DNO and BS7671 figures for 'free air' very similar (at same temp). However, what it doesn't explain is that 'free air' CCCs are the highest per BS7671, but far from the highest for the DNO. I can't imagine that, thermally speaking, 'buried in masonry' can be all that different from 'buried in ground', can it?

Kind Regards, John